Isovolumic times (IVTs) comprise a determinant of exercise capacity in cardiomyopathy. We postulated that an increase in exercise capacity after cardiac resynchronization therapy (CRT) might be related to a more efficient cardiac cycle due to decreasing IVTs and increased filling times. According to standard selection criteria, a CRT device was implanted in 52 patients (37 men; 69 +/- 8 years) with a QRS duration of 174 +/- 30 ms. The etiology was ischemic in 22 and idiopathic in 30 patients. A 6-minute walking test (MWT) and echocardiographic Doppler were performed before and 3 and 6 months after CRT. Timing cycles were obtained with echocardiographic Doppler. An improvement in MWT by >15% (responders) after 6 months of CRT was observed in 46% of patients. The MWT was moderately correlated with baseline time intervals (IVT r = -0.44, filling time r = 0.52), but not to baseline left ventricular ejection fraction (r = -0.06). However, change in the MWT after 3 and 6 months was best related to changes in IVT (r = -0.66 and -0.68, respectively). Receiver-operating characteristic curve analysis of baseline IVT showed that an IVT >29% predicted exercise response with a positive predictive value of 89% and a negative predictive value of 77%. In conclusion, improvement in exercise tolerance after CRT is associated with a decrease in prolonged IVT. Baseline IVT might be used as an adjunctive parameter for selecting symptomatic responders to CRT.